Commercial chlor-alkali membrane cell based installations are efficient in producing elemental chlorine and sodium hydroxide and co-products, such as hydrogen and sodium hypochlorite for commercial sale. However, the transportation and storage of liquid elemental chlorine shipped via tank cars or in one ton cylinders to the point of use at customer sites is increasingly becoming a public safety concern. Rail transportation accidents have occurred in populated areas resulting in the unintentional and sometimes fatal release of the pressurized elemental chlorine gas. In addition, the on-site storage of pressurized elemental chlorine liquid/gas requires costly modifications to facilities to insure secure access and to install caustic scrubbers specifically designed to neutralize unintentional chlorine gas releases. Water and wastewater treatment plants have been converting to the use of sodium hypochlorite, a safer alternative to pressurized elemental chlorine. Bulk delivery and storage of high strength sodium hypochlorite, as well as on-site generation of sodium hypochlorite from salt is increasingly becoming the standard practice for water utilities.
Commercial high strength sodium hypochlorite is typically delivered in a concentration range of 10% to 15% trade to reduce the costs of shipping and handling. In the past 15 years, technology improvements and process economics have evolved to a point where on-site electrochemical generation of high strength sodium hypochlorite is an economically viable alternative to pressurized elemental chlorine and commercially supplied bulk quantities of high strength sodium hypochlorite. In these smaller packaged systems, sodium chloride becomes the primary raw material delivered to the site where it is converted to a sodium hypochlorite solution product.
One of the on-site electrochemical sodium hypochlorite technologies that is presently available generates a 0.8 wt % sodium hypochlorite product solution using a dilute sodium chloride feed solution and an electrochemical cell design that incorporates no separator between the electrolyzer anode and cathode electrodes. The reaction products from the electrodes are allowed to mix together to produce a weak sodium hypochlorite product solution. In this type of system, only 20-30% of the salt in the sodium chloride feed solution is converted to sodium hypochlorite, with the unreacted salt remaining in the sodium hypochlorite solution product. A significant deficiency associated with this technology is the quantity of storage required for the dilute solution product (as compared with high strength sodium hypochlorite) as well as the high salt content of the sodium hypochlorite solution product.
In view of the above, it would be beneficial to develop a system that (a) eliminates the need for transporting liquid elemental chlorine, (b) reduces the need for large sodium hypochlorite storage capacities, and (c) reduces, if not eliminates, the addition of any unreacted feedstock salt into the sodium hypochlorite product.